1. Field of the Invention
The present invention relates to a current-limit system and method, and more particularly, to a current-limit system and method capable of limiting an average current of a DC-DC converter.
2. Description of the Prior Art
DC-DC converters have been widely utilized in power management devices for providing operating voltages with different levels. A well-controlled DC-DC converter can provide a stable output voltage and a wide range of output currents. A conventional DC-DC converter possesses sound protection devices, which not only protect the whole system, but also provide a self-protecting function, such as over-heat protection, current-limit protection, short circuit protection, low voltage lock-out, and high voltage lock-out.
Among these protection devices, the current-limit protecting device, which is capable of limiting an output current of a DC-DC converter when the output current is too large, is one of the most common protecting devices. An inductor type DC-DC converter is provided as an example. Please refer to FIG. 1, which is a schematic diagram of an output circuit 10 of a conventional inductor type DC-DC converter. As shown in FIG. 1, the output circuit 10 includes a driving circuit 106 and an inductor L1. The driving circuit 106 includes a high-side transistor M1 and a low-side transistor M2. The high-side transistor M1, coupled between a power input terminal VCC and the inductor L1, is utilized for supplying charging currents which flow via the inductor L1 to an output terminal OUT. The low-side transistor M2, coupled between a ground terminal GND and the inductor L1, is utilized for providing discharging currents which flow from the inductor L1 to the ground terminal GND. The output circuit 10 may further include a capacitor C1, which is utilized for stabilizing the output voltage.
A conventional current-limit protecting method is performed by detecting the output current of the DC-DC converter or the current flowing through the inductor L1. When the detected current is greater than a predefined value, the system determines that the inductor type DC-DC converter is in an over-current status and enters a current-limit mode. At this moment, the system controls the high-side transistor M1 of the driving circuit 106 to be turned off, in order to prevent the charging current from flowing from the high-side transistor M1 via the inductor L1 to the output terminal OUT. When the current drops to the predefined value or another predefined value, the system determines that the over-current status is released; hence the inductor type DC-DC converter will recover to a normal operating mode. In general, the output current of the DC-DC converter will appear to be a ripple type, as shown in FIG. 2A and FIG. 2B. In FIG. 2A and FIG. 2B, the method of detecting the current is by detecting the peak value of the current. When the peak value of the current reaches the predefined value, the inductor type DC-DC converter will enter the current-limit mode.
The inductor type DC-DC converter experiences various interferences which generate noises. These noises may cause current ripples in the output circuit 10. When the current ripples become greater such that the peak value of the current reaches the predefined value, the current-limit mode may be triggered inaccurately. Therefore, a desirable current detecting method is one which detects an average output current of the DC-DC converter or an average current flowing through the inductor L1. Since the inductors utilized in the DC-DC converters may have different inductances and the output voltages may also be different, the output currents of the DC-DC converter may have different amplitudes. If the peak value is utilized for detecting the current, the corresponding average current may not be obtained accurately. For example, the amplitude of the current in FIG. 2A is smaller, and its peak value is 2 A and average value is 1.8 A; the amplitude of the current in FIG. 2B is greater, and its peak value is 2A and average value is 1.5 A. If the system utilizes a peak current of 2 A as an upper limit of current-limit protection, the system may not possess a current-limit protection with a fixed average current limit. There is therefore a need for providing a current-limit system where the average current is detected as a current limit, in order to prevent the current-limit mode from being triggered inaccurately when noises cause a greater current ripple.